AJR 2004; 182:1477-1484
© American Roentgen Ray Society
Value of Curved Planar Reformations in MDCT of Abdominal Pathology
Terry S. Desser1,
F. Graham Sommer and
R. Brooke Jeffrey, Jr.
1 All authors: Department of Radiology, Stanford University School of Medicine,
300 Pasteur Dr., Mail Code 5621, Stanford, CA 94305.
Received August 25, 2003;
accepted after revision October 10, 2003.
Address correspondence to T. S. Desser.
Introduction
MDCT technology provides rapid coverage of large anatomic regions,
permitting thin slices to be acquired routinely or reconstructed
retrospectively. The resultant thin voxels can then be used to generate
multiplanar displays of anatomy. In many cases, abdominal disease is better
shown in planes other than the axial plane used for native image acquisition.
Planar sagittal, coronal, or oblique views can be easily reconstructed at the
scanner console to supplement the acquisition plane images. These views may be
particularly beneficial in the presence of suspicion of disease oriented in a
craniocaudal direction, such as diaphragmatic hernias. However, a convoluted
loop of intestine or a tortuous vessel typically travels a path that meanders
off the three standard anatomic planes and is rarely restricted to a single
plane.
Curved planar reformations are single images obtained by tracing a curved
path through the imaging volume along the course of a particular anatomic
structure of interest. The volumetric data set is sculpted by a path-tracing
tool into a 2D image representing the undulating plane described by the
structure of interest. The exact algorithm used to create the image can vary,
but in general, once a path has been traced, data points for the image are
generated using linear interpolation in three dimensions. The output is a
single image in which the resultant pixel dimension is equal to the smallest
pixel dimension in the native acquisition.
Curved planar reformations permit even the most tortuous anatomy to be
unwound and displayed along its long axis while preserving the gray-scale
contrast of the original data acquisition. Curved planar reformations can
illustrate complex anatomy and pathology in a single image, thus serving as a
useful communication tool. Our referring clinicians typically want to view
only a few illustrative images of their patients rather than the large data
sets of the native acquisition, and curved planar reformations have become
essential for that purpose. This review will highlight the value of curved
planar reformations in various types of abdominal pathology.
Technique
CT scans were acquired on multidetector scanners (LightSpeed Ultra and
LightSpeed16, General Electric Medical Systems) generating either
eight or 16 slices during each 0.5-sec gantry rotation. In the upper abdomen,
when reformations are planned, images are acquired at 2.5-mm collimation and
then retrospectively reconstructed to 1.25-mm thickness, with 50% slice
thickness overlap (0.6 mm) to eliminate stairstep artifact. For
gastrointestinal tract imaging, patients are instructed to ingest
800–1,000 mL of water, which serves to distend the bowel lumen and
provide contrast with enhancing mucosa. Image acquisition protocols and IV
contrast material administration regimens are optimized for the type of
disorder suspected. Studies are then transferred to a dedicated workstation
(Advantage, General Electric Medical Systems, for Windows, Microsoft), and
curved planar reconstructions are performed by specially trained CT
technologists with 3–12 years' experience in multiplanar reconstruction
techniques [1].
Recently, an algorithm has been developed to automatically generate curved
planar reformations of the vascular system
[2]. The algorithm can rapidly
generate multiple curved planar reformations at 1° intervals rotating
360° along the axis of the path. At present, however, this algorithm
requires high contrast between the target and the background structures and
thus is best suited to vascular applications.
Pancreas and Biliary Tree
Previous work has shown that curved planar reformations can delineate the
course of the pancreatic and bile ducts, illustrating their relationship to
mass lesions occurring in and around the pancreas
[1] (Fig.
1A,
1B). The relationship between
small incidentally detected cystic lesions and main or side branch pancreatic
ducts is important in assessing the clinical significance of these lesions
(Fig. 2A,
2B). Some of these small cystic
lesions represent intraductal papillary mucinous tumors. It has been shown
that intraductal papillary mucinous tumors confined to secondary ducts are
less aggressive than those involving the main pancreatic duct
[3], so establishing the
relationship between these incidental lesions and the main pancreatic duct may
affect clinical treatment. Congenital anomalies of the pancreas, such as
pancreas divisum and annular pancreas (Fig.
3A,
3B), are also well shown.
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Fig. 2A. —58-year-old man with small cystic lesion noted incidentally
in pancreas on prior CT. Axial source image from dedicated pancreatic CT shows
small cystic lesion (arrow), but relationship with main duct cannot
be easily discerned.
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Fig. 2B. —58-year-old man with small cystic lesion noted incidentally
in pancreas on prior CT. Curved planar image shows course of main pancreatic
duct and communication with small cyst (arrow).
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Curved planar images along the length of the common bile duct can depict
intraluminal masses such as stones and papillary tumors, as well as strictures
and ductal cholangiocarcinomas (Figs.
4A,
4B and
5A,
5B).
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Fig. 4B. —72-year-old woman with recent episode of choledocholithiasis.
Curved planar image of common bile duct shows smooth wall thickening
(arrowheads) proximally and intrahepatic biliary dilatation. Entire
common bile duct is visible on single image. Cytology findings were negative,
and biliary dilatation resolved on follow-up CT scan (not shown).
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Stomach and Duodenum
Gastric carcinoma has a dismal prognosis when advanced but is curable in
its early stages. Approximately 10% of gastric cancers are diffusely
infiltrating, although the remaining 90% are equally distributed among the
antrum, the body, and the fundus. Scirrhous cancers most commonly involve the
pylorus and the antrum of the stomach (Fig.
6A,
6B) and are well depicted on
curved planar images. Adequate distention of the stomach with water is
necessary to best show gastric cancers, which can present as regions of focal
or diffuse wall thickening or as polypoid lesions
[4].
Pathology of the duodenum can be difficult to see without a focused
examination and adequate distention of the lumen. Adenocarcinomas, the most
common primary tumors of the duodenum, may occasionally be diagnosed in
patients with suspected pancreatic carcinoma (Fig.
7A,
7B). Stromal tumors of the
duodenum may mimic pancreatic islet cell cancers. Curved planar reformations
can clearly show the duodenal origin of hypervascular periampullary lesions
(Fig. 8A,
8B).
Colon and Appendix
Abnormalities of the colon and the appendix are often well depicted on
axial images but can be difficult to show in their entirety on a single flat
planar image. Curved planar images can highlight the findings of interest so
they are more readily appreciated by referring physicians. We have found
curved planar images to be useful in complicated appendicitis cases (Fig.
9A,
9B) as well as in colorectal
carcinoma (Fig. 10A,
10B).
Urinary Tract
The kidneys and the bladder are commonly well seen in a variety of planes,
but the ureter is rarely seen in its entirety on a single image. Curved planar
reformations can be useful in illustrating the presence of stones
[5] and other intraluminal
lesions in the ureter (Fig.
11A,
11B).
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Fig. 11A. —75-year-old man with right-sided abdominal pain. Axial source
image at level of mid ureter shows right ureteral calculus (arrow).
Additional stone further distal in ureter is not apparent on this image.
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Vascular System
Curved planar reformations found their first application in CT angiography.
Stenoses, thromboses, and aneurysms can be seen with great clarity (Figs.
12A,
12B and
13A,
13B). In addition, curved
planar images can be invaluable for showing vascular involvement or encasement
by malignant processes in the upper abdomen (Fig.
14A,
14B).
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Fig. 12B. —68-year-old woman with acute onset of abdominal pain and
fullness. Curved planar image shows that clot represents thrombosed portal
vein aneurysm with clot involving both portal (white arrow) and
splenic (black arrow) veins.
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Pitfalls
Although curved planar images can clearly depict pathology, they are
geometrically not equivalent to planar images. In space, the shape of the
curved plane has been likened to a magic carpet, meaning innumerable surface
ripples are present. This curved plane is then flattened into a 2D image for
display. Measurements made from curved planar images are typically inaccurate
because of the geometric distortion that occurs at increasing distances from
the centerline. "Loop" or mirror-image artifacts can also occur
when the curved path doubles back on itself. The appearance of the organs
surrounding the structure of interest ranges from slightly to grossly
distorted (Fig. 15A,
15B).
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Fig. 15B. —52-year-old man with pancreatic carcinoma. Curved planar
image along length of splenic artery. When flattened, curved plane described
by looping vessel contains distorted representations of other nearby viscera,
such as artifactually bivalved kidney. Dotted lines indicate mirror-images
artifacts affecting splenic artery.
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Conclusion
Although disease can usually be diagnosed on MDCT from the native axial
acquisition plane or reformatted coronal and sagittal planes, curved planar
reformations can supplement the primary interpretation process and assist in
clarifying complex anatomic relationships. This may be especially helpful in
communicating with referring physicians.
Acknowledgments
We thank Laura Logan, Sean Lee, and the other technologists of the Stanford
University 3D Medical Imaging Laboratory for their work in creating these
curved planar reformations.
References
- Nino-Murcia M, Jeffrey RB Jr, Beaulieu CF, Li KC, Rubin GD.
Multidetector CT of the pancreas and bile duct system: value of curved planar
reformations. AJR2001; 176:689
–693[Free Full Text]
- Raman R, Napel S, Beaulieu CF, Bain ES, Jeffrey RB Jr. Rubin GD.
Automated generation of curved planar reformations from volume data: method
and evaluation. Radiology2002; 223:275
–280[Abstract/Free Full Text]
- Terris B, Ponsot P, Paye F, et al. Intraductal papillary mucinous
tumors of the pancreas confined to secondary ducts show less aggressive
pathologic features as compared with those involving the main pancreatic duct.
Am J Surg Pathol2000; 24:1372
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- Ba-Ssalamah A, Prokop M, Uffmann M, Pokieser P, Teleky B, Lechner
G. Dedicated multidetector CT of the stomach: spectrum of diseases.
RadioGraphics2003; 23:625
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- Sommer FG, Jeffrey RB Jr, Rubin GD, et al. Detection of ureteral
calculi in patients with suspected renal colic: value of reformatted
noncontrast helical CT. AJR1995; 165:509
–513[Abstract/Free Full Text]
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